Manganese dioxide

Manganese dioxide, MnO₂, is a black or dark brown powder made of manganese and oxygen. It occurs in nature as the mineral pyrolusite, and people have used it for thousands of years, from black pigment in cave paintings to clearing the greenish tint from glass.

Battery makers mostly use a very pure form called electrolytic manganese dioxide. Manganese ore is dissolved in sulfuric acid to make a manganese sulfate solution, which is purified. An electric current is passed through it, and manganese dioxide builds up as a hard layer on the electrodes, each manganese atom bound to two oxygen atoms. The deposit is chipped off, ground to powder, washed and neutralized.

In an alkaline battery it is the cathode, the side that takes in electrons. As the battery discharges, the manganese dioxide is reduced, absorbing electrons that have flowed from the zinc through your device. It conducts poorly on its own, so it is mixed with graphite and pressed into a ring inside the can. Good battery-grade powder is dense and very pure: density sets how much energy fits in the can, and impurities make the cell lose charge while it sits on a shelf.